The present invention is directed to bicycle front derailleurs and, more particularly, to a front derailleur with independently movable chain guide plates.
Generally, in cases where a chain is switched among a plurality of front chain wheels by means of a derailleur, it is necessary to cause the chain switching guide to move through a greater lateral stroke in the case of down-shifting, in which the chain is switched from a larger-diameter chain wheel to a smaller-diameter chain wheel by means of the outer chain switching guide of the derailleur, than in the case of up-shifting, in which the chain is switched from a smaller-diameter chain wheel to a larger-diameter chain wheel by means of the inner chain switching guide of the derailleur. Accordingly, in a derailleur in which both the inner and outer guides move as a unit (e.g., in a derailleur in which the outer chain switching guide and the inner chain switching guide are formed as a single part), an up-shifting operation is performed by causing the inner chain switching guide to move through a stroke which is the same as the lateral stroke of the outer chain switching guide in the case of down-shifting. In other words, a shifting operation is performed in this case by causing the inner chain switching guide to complete a stroke which is larger than the stroke actually required for the switching of the chain. Accordingly, in the case of up-shifting, the derailleur is operated while causing a great elastic deformation of the return spring used for down-shifting derailleur operation, so that the shifting operation becomes heavy.
Japanese Patent Application Kokai No. 61-150884 discloses a front derailleur wherein the inner and outer chain switching guides move independently. Specifically, in that derailleur, the inner chain switching guide is connected to the base member via an inner swinging link and an outer swinging link, and a parallel four-member link mechanism is formed by these members. The outer chain switching guide is connected to the outer swinging link, and a motion converting mechanism is coupled to the outer chain switching guide. The motion converting mechanism is constructed so that it converts the movement of a spring which drives the inner chain switching guide toward the low-speed side into a swinging motion of the outer chain switching guide. When an operating arm which is capable of swinging relative to the outer chain switching guide is operated by means of an operating cable, the parallel four-member link mechanism is deformed so that the inner chain switching guide moves in the lateral direction of the bicycle frame. At the same time, as a result of the action of the motion converting mechanism, the outer chain switching guide acts separately from the inner chain switching guide, and moves in the lateral direction of the bicycle frame.
In this derailleur, it is necessary to install the pivoting part, and to install the operating arm separately from the swinging links, etc. Accordingly, the derailleur has a relatively complicated structure.